This invention relates generally to video display controller systems, and, more particularly, it relates to a high resolution graphics display controller system for personal computers.
As the utility and popularity of personal computers has increased, the demand for high quality graphics displays has also increased. When the IBM personal computer (IBM PC) was introduced in 1981, it only produced an 80 character by 25 line monochrome or color text display. A Color Graphics Adapter (CGA) could be added to the IBM PC to generate a 4-color graphics display at a maximum resolution of 320 horizontal picture elements (pixels) by 200 lines (320.times.200), or a 2-color graphics display of 640.times.200. In 1984, the Enhanced Graphics Adapter (EGA) was introduced for use with the IBM PC and compatible computers. Depending on the type fo monitor and the amount of video display memory used with the EGA, graphics displays of up to 16 colors can be produced with maximum resolution of 640.times.350.
The EGA was designed to be "backward-compatible", i.e., it can be configured to work properly with software which was written for the CGA or monochrome adapters, at the lower resolutions generated by those adapters. The EGA has several programmable registers which can be programmed with predetermined combinations of values to configure the EGA in the appropriate mode for the software, the monitor, and the amount of display memory available.
Presently, several companies are marketing video adapters compatible with IBM's EGA, and the EGA has become a de facto standard for display control in IBM and IBM-compatible personal computers. (See "The Enhanced Graphics Standard Comes of Age" PC Magazine Vol. 5, No. 14, August 1986). A standard is important because it allows software writers to focus on creating a single version (or, at least a reasonably limited number of versions) of their software. Consequently, a large body of standardized software is available to consumers.
Monitors are now available for displaying even higher resolution displays than the 640.times.350 graphics produced by current EGAs. There are circuits and computers available which produce higher resolutions. It is desirable, however, to provide higher resolution graphics capability without sacrificing compatibility with the large body of existing software. Hundreds of programs are available for IBM PCs and compatible computers in CGA and EGA modes, and many consumers already own these versions of their favorite programs. Furthermore, it is desirable to take advantage of the low cost of the EGA resulting from its mass production and very high levels of integration. However, higher resolution graphics displays require a graphics adapter circuit which can operate at higher speeds than is currently possible with EGA-compatible adapters.